1. Field of the Invention
The present invention relates generally to a mold for molding disc substrata used for video discs, compact discs, or memory discs for digital data (all hereinafter described as discs) and particularly concerns a novel mold in which a stamper plate is capable of being set selectively on either a stationary half-mold side or a movable half-mold side.
2. Description of the Related Art
Generally, a mold for molding discs substrata provides a stationary half-mold including a stationary mirror plate and a movable half-mold including a movable mirror plate.
The stationary half-mold is fixed to a stationary platen of an injection molding machine, wherein a nozzle is capable of touching a sprue bushing inserted into a central portion of the stationary mirror plate so as to inject melted resin into a cavity. The movable half-mold is fixed to a movable platen, wherein a punch for forming a central hole of the disc, an ejector sleeve for ejecting a solidified disc, and an ejector pin for ejecting a solidified sprue are conjointly provided as an assembly in the central portion of the movable half-mold, and a stamper plate in which information digital data are engraved is retained on the movable mirror plate by an inner peripheral stamper holder and an outer peripheral stamper holder.
Generally the stamper plate has been fixedly set on either side, the stationary or the movable half-mold, in accordance with an original design of the mold.
In injection molding of the discs, the flow behavior of melted resin has an important effect on molded discs. Therefore it is significant to change the flow behavior of melted resin in a mold cavity of the discs.
A following method is given as a concrete example.
In a mold for molding discs, it is suggested that a stamper plate is mounted not only on a movable plate but on a stationary plate. Generally the stamper plate is mounted on a cavity surface of the movable plate side which faces a cavity of the stationary plate side having a sprue bushing for injecting of melted resin. It is a new method for changing the flow of melted resin that the stamper plate is mounted on the cavity surface of the stationary plate side which the sprue bushing is positioned.
However, if changing the set position of the stamper plate is required, many parts of the mold must be modified after dismounting of the mold from the injection molding machine.
Such modification increases molding cost in respect to the additional design and manufacturing of the mold parts.